Further studies have been carried out with a restriction system of H. influenzae that requires ATP and SAM. The enzyme is specific for unmodified DNA, and can recognize phage or bacterial DNA. Two different ATP and SAM requiring enzymes have been isolated from H. influenzae type b. The b restriction system which is stable in b cells has been transformed to H. influenzae type d and shown to be stable in this background as well. These data suggest that the d restriction and modification system in some way regulates itself. The genetic map of the H. influenzae has been improved and markers can now be routinely mapped. The methylases of H. influenzae have been isolated and partially purified, and the methylase responsible for protection against H. influenzae 'endo R has been identified by its ability to prevent inactivation of H. parainfluenzae transforming DNA by endo R of H. influenzae.